class MyWindow: public WithMyCustomLayout<TopWindow> {
...
   Callback WhenMinimize; // callback for minimize

   // Overriding default WindowProc
   LRESULT WindowProc(UINT message, WPARAM wParam, LPARAM lParam);
...
};

// implementation of WndProc
LRESULT MyWindow::WindowProc(UINT message, WPARAM wParam, LPARAM lParam)
{
	if (message == WM_SYSCOMMAND && wParam == SC_MINIMIZE)
	{
		WhenMinimize(); // callback
		return 0;
	}
	return TopWindow::WindowProc(message, wParam, lParam);
}

// in window constructor or any other suitable method, 
// assign custom function to callback
MyWindow::MyWindow()
{
...
   WhenMinimize = THISBACK(MyCustomFunction);
...
}